Phthalocyanine pigments are widely used for the production of printing inks. In their crude form, such pigments are in a crystalline, particulate state, with the individual crystals having a generally elongated, needle-like configuration. In the case of so-called crude green shade phthalocyanine blue, the crude pigment employed for production of blue-shaded inks, the crude pigment particles typically have dimensions on the order of approximately 3 microns by 40 microns.
In its crude form, phthalocyanine pigments do not have the requisite strength or color value needed for the production of printing media. The crude pigment must therefore be "developed" by significantly reducing the particle size to bring out the desired color characteristics and strength of the pigment. In the industry, this development is referred to as being effected by changing the crude pigment from its "alpha" state to its "beta" state wherein the particle size is significantly reduced. When this "flop" in the pigment state is effected, particle size is preferably on the order of 0.4 microns or less (with the particles being roughly spherical), although size reduction to a minimum of one micron or less can be satisfactory.
The most commonly employed technique for effecting the desired reduction in phthalocyanine pigment particle size involves the viscous grinding of crude phthalocyanine particles. The crude pigment particles are combined with a grinding medium, typically comprising microsized sodium chloride salt, in a large double-arm mixing device. Diethylene glycol is also combined so as to create an amorphous, viscous mass, with the glycol also used for controlling the temperature of the mass during grinding. Relatively large amounts of salt are used in the process, typically on the order of 5.7-to-1 to 8-to-1 parts of salt to crude pigment, by weight. The amount of glycol typically employed is on the order of 1.4-to-1, glycol to crude pigment by weight.
The mass thus formed is mixed in the grinding device for an extended period of time, typically on the order of 8 to 10 hours. After viscous grinding in this manner, the conventionally acceptable desired reduction in the particle size of the pigment has been effected, with the resultant mass referred to as "magma". The magma is then combined with large quantities of water to form a slurry and transferred to a boiling tank. Hydrochloric acid is added to dissolve the impurities in the pigment, with the mixture boiled at approximately 90 degrees centigrade for about one and one half hours.
The slurry is then pumped into a filter press which captures the pigment particles and allows the water carrying the dissolved salt and glycol to pass through for disposal. In conventional processes, this step takes approximately two and one-half hours. The filter press is then backwashed for a period of approximately 4 to 7 hours, depending upon the quantity of salt originally employed in the grinding process. The filter press is then emptied and the resultant "press cake" of finished pigment is emptied having a content of approximately 35 percent pigment and 65 percent water.
While the above-described process has been almost universally employed for years for production of refined or developed phthalocyanine pigments, the shortcomings of the process will be readily appreciated. Relatively large amounts of both salt and glycol are required for the viscous grinding, with the grinding itself being both time-consuming and energy-intensive. Because of the relatively large amounts of salt used, the filtering and backwashing steps are also time-consuming, requiring relatively large quantities of fresh water. While the resultant pigment product is of a quality which has found acceptance by the printing industry, testing during the development of the present invention has shown that the resultant pigment is "under-developed", and that its full tinctorial characteristics have not been realized.